The present invention relates to an image forming apparatus for forming an image on a photosensitive body by using a liquid crystal shutter comprising a guest-host effect type liquid crystal cell and for transcribing the image to a recording medium.
A conventional apparatus having a liquid crystal cell as a shutter is known as an electrophotographic printer described in U.S. Pat. No. 4,386,836. According to this electrophotographic printer, light from a light source irradiates a uniformly charged photosensitive drum upon energization of a charger through a twisted nematic mode liquid crystal cell (to be referred to as a TN liquid crystal cell hereinafter), and so forms a latent image corresponding to an image of a document on the photosensitive drum. The latent image is developed and transcribed to the recording medium.
The TN liquid crystal cell has the structure as shown in FIG. 1. Two glass substrates 3A and 3B are opposite to each other, and transparent electrodes 3C and 3D are respectively formed on the inner surfaces of the opposing glass substrates. In this case, the transparent electrodes 3C serve as signal or segment electrodes, and the transparent electrode 3D on the glass substrate 3B serves as a common electrode. Opaque electrodes 3F made of a metal such as chrominum are formed on the transparent electrode 3D. An aligning film 3J is formed on exposed portions of the glass substrate 3A and the transparent electrodes 3C. An aligning film 3K is formed on the exposed portions of the transparent electrode 3D and the opaque metal electrodes 3F. A polarizing plate 3G is formed on the outer surface of the glass substrate 3A. Similarly, a polarizing plate 3H is formed on the outer surface of the glass substrate 3B. The polarizing plates 3G and 3H constitute crossed nicols. A double-frequency driven liquid crystal 3I is sealed in a space between the aligning films 3J and 3K, thereby preparing a liquid crystal shutter. When a voltage of low frequency is applied across the electrodes 3C and 3D through the liquid crystal 3I, light is not transmitted through the electrodes 3C and 3D, so that the portions indicated by reference numeral 3E serve as shutters, as shown in FIG. 2. The liquid crystal shutter has the double-frequency driven liquid crystal whose dielectric anisotropy is inverted at a frequency fC, as shown in FIG. 3. The double-frequency driven liquid crystal comprises a nematic liquid crystal whose cross over frequency fC (at which the dielectric anisotropy becomes zero) is less than 100 kHz at ordinary temperature. This nematic liquid crystal contains an optically active substance. The direction of dielectric anisotropy is positive at a frequency fL which is lower than the cross over frequency fC. The direction of dielectric anisotropy becomes negative at a frequency fH which is higher than the cross over frequency fC. In the double-frequency driven liquid crystal having the property described above, when a low-frequency (fL) voltage is applied across the electrodes, the liquid crystal molecules are aligned parallel to the direction of an electric field. However, when a high-frequency (fH) voltage is applied across the electrodes, the molecules are aligned perpendicular to the direction of the electric field. Since the polarizing plates 3G and 3H are arranged such that their polarizing axes are orthogonal to each other, the shutter portions 3E shield light when the low-frequency (fL) voltage is applied across the electrodes. However, the shutter portions 3E transmit light when the high-frequency (fH) voltage is applied across the electrodes.
In the liquid crystal shutter having the construction described above, the hatched portion in FIG. 2 can always transmit light. The light component from the hatched portion irradiates the photosensitive drum, which disables the formation of a desired latent image thereon. In order to solve this problem, a mask is used to cover the hatched portion. However, manufacture of the mask complicates the fabrication process, and the construction of the liquid crystal shutter also becomes complicated. In particular, when the mask is prepared by using an opaque electrode made of a metal such as chrominum, the opposing area between the mask electrode and the lead electrode is increased, and the capacitance therebetween is increased. As a result, the power consumption of the liquid crystal shutter is increased, resulting in an economical disadvantage.
In addition, the TN liquid crystal cell used in the conventional electrophotographic printer contains an optically active substance such as a chiral nematic liquid crystal so as to shorten the response time. This leads to the following drawback. Optically active substance is contained in the TN type cell, so that the liquid crystal molecules are twisted by between about 270 degrees and 450 degrees. The twist angle changes in accordance with a gap between the glass substrates of the liquid crystal cell. High-precision control of the twist angle is therefore demanded. In addition, when the liquid crystal molecules are twisted and applied with a voltage, and the molecular axes of the liquid crystal are homogeneously to the direction of the electric field, the response time of the cell is short. However, when the liquid crystal molecules are deenergized and returned to the initial alignment, the cell has a long response time. In other words, the response time for turning off the microshutter includes a time delay for twisting the liquid crystal molecules.
Furthermore, since the twist angle of the liquid crystal molecules is as large as 270 degrees to 450 degrees, the vibration plane of light transmitted through the cell which light has been polarized by one polarizing plate is turned in accordance with the twist angle. Since the cell has a large twist angle, light scattering becomes large, thereby decreasing the contrast. For this reason, a high-intensity light source must be used under the assumption that light is greatly scattered. In addition to this disadvantage, when a temperature of the liquid crystal material changes, the twist angle of the liquid crystal molecules also changes, thereby destabilizing the contrast and the response time. Therefore, a temperature control device must be used to precisely control the temperature of the liquid crystal cell.